Geologically, locations with volcanic activity - past and present - lend themselves most easily to the harnessing of geothermal heat. This is because the warm, or even hot, water comes to the surface in most cases naturally such as e.g. springs, geysers, etc.
Northern Europe
Iceland is the exception in northern Europe. Geothermal energy has long been used there, as for example in Reykjavik - because of its unique location, a place on the edge of the Eurasian and North American tectonic plates. Along such boundaries, you see volcanic or seismic activity all over the world because these plates move.
So, in theory, you can most easily extract geothermal heat there.
Central Europe
Most of Europe lies as one unit on the Eurasian tectonic plate, so in the centre of Europe there is less volcanic activity. However, this does not mean that there are no heat basins suitable for geothermal extraction.
There are high-temperature basins in France, Germany and Hungary. Hence, France discovered the benefits of geothermal heat several centuries ago.
Furthermore, there are hotspots from Ireland to Romania, with sometimes very large areas as in eastern Europe, in Ukraine and Belarus. The heat basin to which Belgium and the Netherlands belong also stretches across central and eastern Europe to the Black Sea. So here is potential to engage in geothermal extraction. The how further depends on geological factors.
Southern Europe
Southern Europe, in turn, borders the end of the tectonic plate. So it is no wonder that Italy also has a volcanic region. The east coast of Italy in particular is ideal for geothermal applications, if of course the other conditions can be met. Heat does not automatically mean there is enough water present to do deep geothermal. The other southern European countries follow the same concept as central Europe: clusters of high and medium temperature basins.
